Silver halide emulsions are generally prepared using a reactive precipitation process; aqueous solutions of silver nitrate and alkali halides are reacted in the presence of gelatin. The composition of resultant product (silver halide emulsions) is tuned by varying the constituents of the alkali halide solution. For example, the precipitation of silver bromide emulsions is carried out using sodium bromide as the alkali halide, while silver chloride emulsions are precipitated using sodium chloride. Appropriate addenda/dopants are generally introduced as aqueous solutions during the precipitation process, to generate silver halide emulsions of desired composition.
The important feature of all these processes is the bimolecular chemical reaction between (Ag+) ions and the appropriate anion(s) to generate the precipitating species. It is possible to vary the chemical and the structural composition of the product emulsion by varying the constituents of the reagent solutions, but the chemical reaction responsible for the generation of the desired silver halide emulsion is always the reaction between (Ag+) ions that are present in a solution or on the surface of the silver halide emulsion, and the appropriate anion(s).
From an operational point of view, generation of silver halide emulsions by this reactive precipitation process involves the addition of concentrated reagent solutions into a reactor under vigorous mixing conditions. The goal of the mixing process is to minimize the volume of the reactor that is exposed to the unreacted reagent solutions. However, even under ideal mixing conditions, the volume of the reactor that is exposed to the unreacted reagents is finite and relatively large.
In order to understand the reasons for the exposure of the reactor contents to unreacted reagents it is necessary to examine the mechanism of the mixing process. Mixing in emulsion precipitation processes is achieved by means of a rapidly spinning rotary agitator. The momentum generated by the rotary agitator results in the circulation of the fluid in the reactor. Appropriate baffling devices are used to randomize the fluid motion in the reactor, to achieve efficient mixing. It is important to recognize that efficient mixing requires rapid circulation of the fluid in the reactor. In a typical emulsion generation process, the reagent solutions are introduced into a region of the reactor that experiences good mixing. Consequently, the concentrated reagent solutions are introduced into a region of the reactor that experiences rapid circulation of the fluid in the reactor; i.e. the reagent introduction region in the reactor is exposed frequently to the contents of the reactor.
It is also important to recognize that efficient mixing is necessary at the reagent introduction region, in order to promote the reaction between the concentrated reagents. Because this (efficient) mixing process is carried out by rapid circulation of the reactor fluid through the reagent introduction region, the contents of the reactor are necessarily exposed to the concentrated reagents. From a kinetic view point, the extent of exposure of the reactor contents to the unreacted reagents would depend on the rate of dilution of the concentrated reagents relative the rate of the chemical reaction between the concentrated reagents. Under ideal mixing conditions, the rate of dilution of the concentrated reagents is determined by the molecular/ionic diffusivity of the reactant species; which is still considerably smaller than the rate of the relevant chemical reactions. Hence, the extent of exposure of the reactor contents to the unreacted reagents can be significant even under ideal mixing conditions.
The unintentional exposure of the reactor contents to the unreacted reagents can have undesired effects on the emulsion crystals. For example, exposure of unreacted silver nitrate can result in the creation of fog centers in the crystals.
Furthermore, consistent nucleation conditions are critical to repeatable precipitations. Double jet nucleation using solutions of silver nitrate and soluble halides, depend critically on matched reagent arrival times in this reactor. Small variations in the mixing of the reactants upon arrival in the reactor can result in high levels of variability in the final emulsion grain size or size distribution. Hence, nucleation is generally carried out under conditions of relatively high halide concentrations (high pAg) in order to ameliorate pAg shifts associated with any mismatch in the solution properties or arrival times.
The use of concentrated solutions of silver halide complexes prepared from methylamineformamide and excess halide have been reported as a method for alleviating these concerns. However, methylamineformamide is exceedingly hazardous, and the solvent has been documented as a teratogen (promotes deformity in embryos).